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Goldfarb, R.J., 1990

Genesis of lode gold deposits of the southern Alaskan Cordillera

Bibliographic Reference

Goldfarb, R.J., 1990, Genesis of lode gold deposits of the southern Alaskan Cordillera: University of Colorado, Boulder, Ph.D. dissertation, 437 p., illust., maps.


Mesothermal gold-bearing quartz veins are widespread throughout the allochthonous tectonostratigraphic terranes of southern Alaska. The host terranes are largely composed of complexly deformed, clastic metasedimentary rocks, with lesser felsic to mafic volcanic rocks, and post-accretionary, calc-alkaline intrusive bodies. Auriferous veins and related placers are restricted to medium-grade metamorphic sequences. The veins are found in both small shear zones and tensile fractures, and along major crustal lineaments. Gold deposits were studied within the Port Valdez, Port Wells, Girdwood, Moose Pass, and Hope-Sunrise districts in south-central Alaska, and in the Chichagof district and along the Juneau gold belt in southeastern Alaska. Pyrite, pyrrhotite, arsenopyrite, galena, sphalerite, and/or chalcopyrite occur, along with gold, in these systems. Wall rock carbonatization, sulfidization, silicification, and less commonly, sericitization or albitization, are consistent features in all of the gold districts. Fluid inclusion examination by microthermometry, laser Raman spectroscopy, and mass spectrometry, along with stable isotope studies, constrained ore fluid sources and conditions of ore deposition. Homogenization temperatures for ore-related fluid inclusions ranged between 150 and 310 degrees C, with the majority between 210 and 280 degrees C. Trapping pressures are estimated to be at least 1-2 kb for all deposits, corresponding to depths of at least 3.5 to 7 km. Most ore solutions, trapped in the one-phase field, contained less than 5 eq. wt% NaCl and consisted of at least 90 mole percent H2O, plus lesser CO2, CH4, N2, AND H2S. The ore fluids at the Alaska-Juneau deposit, however, contained approximately 50 mole percent CO2 and showed unequivocal evidence of fluid immiscibility. Calculated isotopic compositions for ore fluids at all deposits ranged from 6-10 per mil for delta 18O and 20-30 per mil for delta D, consistent with metamorphic water. The data are incompatible with deeply circulating meteoric fluids, unless enormous exchange occurred between the hydrothermal fluids and the metasedimentary pile. Absolute and relative age relationships indicate gold genesis during the Eocene and Oligocene throughout much of southern Alaska. The ore solutions were derived from devolatilization of dominantly pelitic, subducted crust. These fluids migrated upward to higher, retrograding custral levels, that were undergoing rapid uplift, prior to precipitation due to oxidation and sulfidization reactions.

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